The present invention relates to rotary fluid pressure devices, and more particularly, to valving for such devices which results in substantially improved low-speed operation.
Although the invention may be used with devices having various types of fluid energy-translating displacement mechanisms, the invention is especially adapted for use in a device including a gerotor gear set, and will be described in connection therewith.
Fluid motors of the type utilizing a gerotor gear set to convert fluid pressure into a rotary output have become popular and are especially suited for low-speed, high-torque applications. In one of the most common designs of such motors, the housing defines inlet and outlet ports and a cylindrical valve bore, and the motor further includes a hollow, cylindrical spool valve which is integral with an output shaft. The well known commutating valve action necessary to communicate pressurized fluid to the expanding volume chambers of the gerotor set, and communicate exhaust fluid from the contracting volume chambers, occurs at the interface of the valve bore and the valve spool.
As is well known to those skilled in the art, commutating valving for use with a low-speed, high-torque (LSHT) gerotor motor requires a stationary valve member and a rotary valve member. Typically, if the gerotor gear set includes N+1 internal teeth and N external teeth, the stationary valve will define N+1 ports (each of which communicates with one of the volume chambers of the gerotor), and the rotary valve defines N fluid ports (in communication with the pressurized motor inlet). Fluid motors and commutating valving of the type to which this invention relates are illustrated and discussed in greater detail in U.S. Pat. No. 3,514,234, assigned to the assignee of the present invention and incorporated herein by reference. Typically in such motors, both the stationary ports and the rotary valve ports have a port width W.sub.1. At the same time, the sealing lands between the ports on the rotary valve have a width which is also equal to W.sub.1. Such an arrangement is referred to as "zero lap porting," i.e., a stationary port can be disposed between adjacent high-pressure and low-pressure rotary ports, and in line-to-line contact with each, without actually being in fluid communication with either (see FIG. 2). In an increasing number of applications for LSHT gerotor motors, it has become desirable to operate the motor at extremely low flows and speeds, such as several rpm. Prior to the present invention, when LSHT gerotor motors were operated at such low speeds, the motor either stalled or operated with a very uneven output speed. It is believed that such operation is the result of cross-port leakage (i.e., leakage from a pressurized rotary port through an adjacent stationary port and into the adjacent low-pressure rotary port). With zero lap porting as used in the prior art, there is sufficient time for such cross-port leakage to occur while the motor is operated at very low speeds.